On 02/01, Oleg Nesterov wrote:
>
> > + onsigstack = on_sig_stack(sp);
> > + if (ss_size == 0) {
> > + switch (ss_flags) {
> > + case 0:
> > + error = -EPERM;
> > + if (onsigstack)
> > + goto out;
> > + current->sas_ss_sp = 0;
> > + current->sas_ss_size = 0;
> > + current->sas_ss_flags = SS_DISABLE;
> > + break;
> > + case SS_ONSTACK:
> > + /* re-enable previously disabled sas */
> > + error = -EINVAL;
> > + if (current->sas_ss_size == 0)
> > + goto out;
> > + break;
> > + default:
> > + break;
> > + }
>
> and iiuc the "default" case allows you to write SS_DISABLE into ->sas_ss_flags
> even if on_sig_stack().
>
> So the sequence is
>
> // running on alt stack
>
> sigaltstack(SS_DISABLE);
>
> temporary_run_on_another_stack();
>
> sigaltstack(SS_ONSTACK);
>
> and SS_DISABLE saves us from another SA_ONSTACK signal, right?
>
> But afaics it can only help after we change the stack. Suppose that SA_ONSTACK signal
> comess before temporary_run_on_another_stack(). get_sigframe() should be fine after
> your changes (afaics), it won't pick the alt stack after SS_DISABLE.
>
> However, unless I missed something save_altstack_ex() will record SS_ONSTACK in
> uc_stack->ss_flags, and after return from signal handler restore_altstack() will
> enable alt stack again?
OK, I didn't notice you modified save_altstack_ex() to use ->sas_ss_flags instead
of sas_ss_flags()... still doesn't look right, in this case restore_altstack() will
not restore sas_ss_size/sas_ss_sp and they can be changed by signal handler.
Anyway, whatever I missed I agree with Andy, SS_FORCE looks simpler and better to me.
Oleg.
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